1. Field of the Invention
The present invention relates to a pad conditioner, a pad conditioning method, and a polishing apparatus, in particular, a pad conditioner which recovers a surface of a polishing pad in a polishing apparatus for polishing a work such as a semiconductor wafer, a pad conditioning method, and a polishing apparatus which is provided with the pad conditioner.
2. Description of the Related Art
As microstructure and multilayered structure of a semiconductor device have been achieved, CMP (Chemical Mechanical Polishing) technology has become essential in a manufacturing process of the semiconductor device. The CMP technology is now used for planarization of an insulating film between layers, as well as in various processes of Cu wiring and isolation of elements, for example.
In the CMP technology for planarization, removal rate uniformity (polishing uniformity) across a work surface is an important specification. In order to improve the polishing uniformity, it is important to uniformly distribute any elements which affect the removal rate across a work surface.
While such important elements include a polishing pressure and a relative velocity in polishing, a surface state of a polishing pad is also an important element quantification of which is still in a preliminary stage. A preferable surface state of a polishing pad is achieved by conditioning the polishing pad. Also, a fact that, in a so-called in-situ conditioning in which a pad is conditioned during polishing, for example a stopping of the conditioning suddenly drops a removal rate, shows that a precise control of a surface state of a polishing pad is important.
Pad conditioning is an act to bring a pad conditioner (hereinafter, it may be simply referred to as a conditioner) having grinders such as diamond into contact with a polishing pad to scrape or roughen a surface of the polishing pad, so that a surface state of a new polishing pad is optimized as an initial state with a good holding capacity of the slurry or the slurry holding capacity of the polishing pad in use is recovered to maintain its polishing capability.
Conventionally, pad conditioners having diamond abrasive particles electro-deposited thereto have been often used to condition a polishing pad by being pressed against the polishing pad while rotating around its central axis (for example, see Japanese Patent Application Laid-Open No. 2001-274122 and corresponding U.S. Pat. No. 6,623,344, or Japanese Patent Application Laid-Open No. 2003-181756).
FIG. 12 is a conceptual view to illustrate a pad conditioner described in Japanese Patent Application Laid-Open No. 2001-274122 and corresponding U.S. Pat. No. 6,623,344. The pad conditioner 130 described in Japanese Patent Application Laid-Open No. 2001-274122 and corresponding U.S. Pat. No. 6,623,344 includes, as shown in FIG. 12, a substrate 131 having diamond abrasive particles 133 electro-deposited thereto, and a supporting section 132 to which the substrate 131 is fixedly attached.
FIG. 13 is a conceptual view to illustrate a pad conditioner described in Japanese Patent Application Laid-Open No. 2003-181756. The pad conditioner 130A described in Japanese Patent Application Laid-Open No. 2003-181756 in principle includes a substrate 131A, diamond abrasive particles 133 which are electro-deposited to the substrate 131A, and a supporting section 132A, and the substrate 131A is swingably mounted to the supporting section 132A in every direction, via a so-called gimbal arrangement of a ball joint 132a for example, so as to follow a surface of a polishing pad 20.
Apparatuses having a brush conditioner in addition to a diamond conditioner are also known (for example, see Japanese Patent Application Laid-Open No. 2003-211355 and corresponding U.S. Pat. No. 6,953,390). Japanese Patent Application Laid-Open No. 2003-211355 and corresponding U.S. Pat. No. 6,953,390 disclose a diamond conditioner for scraping a polishing pad surface as a first conditioner and a brush conditioner for extracting foreign matters clogged in recesses in the polishing pad surface as a second conditioner.
Since the brush conditioner is for extracting foreign matters clogged in recesses in the polishing pad surface, it is the diamond conditioner of the first conditioner which conditions the pad.
The brush conditioner has a nylon brush as a brushing member. The nylon brush can brush a pad surface, but is not effective for scraping the pad surface.
Thus, in the technology disclosed in the above document, the brush conditioner is used as a device only for removing foreign matters in a pad surface, not for scraping and conditioning the pad surface with a brush.
Other than the above, there is another document which describes about conditioning by use of brush (for example, see the above Japanese Patent Application Laid-Open No. 2003-181756 or Japanese Patent Application Laid-Open No. 10-329003). However, both brushes described in Japanese Patent Application Laid-Open No. 2003-181756 and Japanese Patent Application Laid-Open No. 10-329003, as well as the one in Japanese Patent Application Laid-Open No. 2003-211355 and corresponding U.S. Pat. No. 6,953,390, are described to use in a brushing method for extracting foreign matters in a pad surface, not to use in a conditioning method for scraping a surface of a polishing pad.
A polishing pad used in a CMP apparatus has a surface which is not flat when the pad is attached to a polishing plate, because the thickness of the polishing pad itself is uneven or the polishing pad is attached to the polishing plate unevenly. A surface of a polishing pad attached to a polishing plate usually has a height difference of 30 μm to 50 μm.
However, in CMP, in order to uniformly polish a wafer surface, a uniform conditioning over a polishing pad surface having such unevenness is required by following such a surface with a pad conditioner.
FIG. 11 is a view to show a concept of a pad conditioning specification required in CMP. As shown in FIG. 11, when a polishing pad 20 having the waviness with a height difference of 50 μm for a width of about 100 mm is conditioned for example, a uniform conditioning by following the waviness is required. Since a polishing pad is made of an elastic material in this way, such a pad conditioning in a CMP apparatus is considered to be a reference grinding process on an elastic material.
Whereas, the pad conditioner 130 described in Japanese Patent Application Laid-Open No. 2001-274122 and corresponding U.S. Pat. No. 6,623,344 is completely fixed to the supporting section 132, and this configuration allows only the top portion of a waviness of a polishing pad surface to be scraped. Thus, the pad conditioner 130 has a problem that a uniform conditioning by following a polishing pad surface cannot be achieved.
The pad conditioner 130A described in Japanese Patent Application Laid-Open No. 2003-181756 has a conditioning surface which is supported to follow a polishing pad surface, but in an actual pad conditioning, a uniform conditioning by following a polishing pad surface cannot be achieved. This is because the pad conditioner is positioned at an angle to the polishing pad 20 due to a large frictional force applied to the pad conditioner surface in contact with the polishing pad 20 which is moving at a high speed. The positioning at an angle makes the frictional force to be decreased, and the pad conditioner returns to its original posture, thereby the pad conditioner discontinuously contacts the polishing pad 20 (stick-slip).
The following problems on the polishing performance of the wafer are caused by the ununiformity (unevenness) of the conditioning in the polishing pad surface. The polishing ununiformity (polishing unevenness) is occurred because there are a conditioned portion and an unconditioned portion in the polishing pad surface. Next, in the step of stabilizing the polishing rate, the entire polishing pad surface is not conditioned evenly and the conditioning is made to partially progress, so that stabilizing the polishing rate takes long time. The polishing pad in which the polishing rate is yet stabilized, can not subject a product wafer to the polishing processing. As a result, the startup time of the polishing pad takes long.
In case of pad conditioning is an act to bring the conventional plate type pad conditioner to which diamonds are electro-deposited into contact with a polishing pad, the pad conditioner is positioned at an angle to the polishing pad due to a large frictional force applied to the pad conditioner surface in contact with the polishing pad which is moving at a high speed. The positioning at an angle makes the frictional force to be decreased, and the pad conditioner returns to its original posture, thereby the pad conditioner discontinuously contacts the polishing pad. In this way, the conditioning of the polishing pad in a circumferential direction is performed unevenly. The problems caused by the uniformity of the conditioning are not limited to the above described.
The size of the scrapes of the polishing pad scraped by conditioning varies greatly, since the portions of small scrapes and large scrapes are generated because the pad conditioner discontinuously contacts the polishing pad. The surface of the polishing pad is scraped as exfoliated a big pieces not stably scraped by small amount, accordingly, the scrapes volume of the polishing pad by the conditioning enlarges. As a result, the consumption amount of the pad surface is increased, leading to a problem that the life of the polishing pad is short and then the replacement cycle of the polishing pad is also short. In this way, conventional pad conditioners have an essential problem in their structures from the point of view of a reference grinding over an elastic body.
Prior to the present invention, the inventor of the present invention made a study to assess the effect of a pad conditioning. First, a recovery of a removal rate of a clogged polishing pad was evaluated by brushing a surface of the polishing pad with a nylon brush while pure water being supplied to the pad without scraping the polishing pad surface. As a result, the removal rate recovered not more than 31.4% in spite of the pure water supply and brushing for a long time of period, where it was observed under SEM that foreign matters in the polishing pad surface were completely removed from the polishing pad surface (related document: Daichi Kamikawa and Takashi Fujita et al., Proceedings of the 2004 Japan Society for Precision Engineering Conference, Tohoku Regional Branch, p. 22).
The above experiment shows that such a brushing only removes foreign matter remaining on a surface of a polishing pad, but does not condition a pad. The inventor of the present invention confirmed that the removal rate recovers with a usual conditioning using diamonds for scraping the polishing pad surface after the brushing, and concluded that a scraping of a polishing pad surface is necessary for a pad conditioning.
In addition, the inventor of the present invention verified that a surface of a polishing pad is chemically modified when a removal rate of the polishing pad is decreased due to clogging. The inventor of the present invention also confirmed that scraping the diverse material partially recovers the removal rate. This result suggests that a removal rate decreases not only when foreign matters are deposited in pores of a polishing pad, but also when the polishing pad surface chemically modifies (related document: Takashi Fujita, Proceedings of the 2005 Japan Society for Precision Engineering Conference, Spring, p. 845).
In this way, the inventor of the present invention confirmed that extracting foreign matters in a polishing pad surface is not sufficient as a conditioning to maintain a removal rate, and scraping the modified pad surface is essential.
A conditioning in terms of scraping a polishing pad surface is already achieved with a conditioning plate of the first conditioner to which diamonds are electro-deposited, in the above Japanese Patent Application Laid-Open No. 2003-211355 and corresponding U.S. Pat. No. 6,953,390, for example.
The above description shows that a pad conditioning which includes scraping a polishing pad surface by following a surface of the polishing pad, that is, finely grinding the polishing pad is indispensable.
However, a pad conditioning is not just a scraping of a polishing pad surface. Roughening the polishing pad surface during the scraping is also necessary, and when the surface is carved with a tool such as a plane without roughening, the slurry holding capacity of the polishing pad is degraded, which is not a result of conditioning to obtain. In conditioning, it is important to scrape a pad surface while a microscopically rough surface being formed which is required to improve or maintain the slurry holding capability of the pad.
When a finely-scraped, microscopically rough surface of a polishing pad is achieved by conditioning, the modified portion of the polishing pad surface is effectively removed, and a substantial surface area for holding slurry is increased in the polishing pad surface, which allows a sufficiently high removal rate to be ensured.
Because the polishing pad is an object to be processed which is made of a polymer composite resin material and of a material having a number of air bubbles in its surface, the process to achieve a microscopically rough surface of a polishing pad is very different from a grinding in a usual metal processing or ceramic processing. In grinding a porous resin material with abrasive particles, a size of the abrasive particles, a tip shape of the abrasive particles, and the like should be carefully designed so as not to tear off the surface by the particles cutting into the surface.
In a conventional pad conditioning, a plate to which diamonds are electro-deposited is pressed against a polishing pad. As a result, a polishing pad surface is scraped by the moving diamonds of the plate. However, since the diamonds electro-deposited to the plate do not cut into the pad enough under a low pressure, a somewhat higher pressure is applied to press down the plate for conditioning. Therefore, the plate roughens a polishing pad surface while grinding, but eventually it scrapes the surface too much due to the higher presser, and shortens the life of the polishing pad.
Prior to the present invention, the inventor of the present invention first roughened a polishing pad surface with a tool having a tiny needle to which diamonds are electro-deposited. The inventor confirmed a result that the surface was quite effectively scraped, and observed under SEM that the surface was effectively microscopically roughened despite the surface of the polishing pad being also a porous resin material.
However, as described above, in the case of a conditioner using a conventional conditioning plate, the conditioning plates is positioned at an angle to a polishing pad due to a frictional force between the polishing pad and the conditioner, and the positioning at an angle makes the frictional force to be decreased, and the pad conditioner returns to its original posture, thereby a conditioning is performed by the pad conditioner which discontinuously contacts the polishing pad.
With respect to the discontinuous contact, a report about an actual research has been issued (See Ara philipossian, Zhonglin Li, Hyosang Lee, Len Borucki, Ryozo Kimura, Naoki Rikita and Kenji Nagasawa, Effect of Diamond Disc Conditioner Design and Kinematics on Process Hydrodynamics during Copper CMP, Proceedings of CMP-MIC conference (2005) p. 43, T. P. Merchanr, J. N. Zabasajja, L. J. Borucki, A. Scott Lawing, A Pad Wear Model For CMP Process Optimization, Proceedings of CMP-MIC Conference (2005) p. 143-150).
In the report, a tilting of a pad is mentioned in which a conditioner tilts relative to a pad due to a frictional force when a small vertical load is applied to a conditioner, and the moving of a conditioning plate is analyzed in consideration of the tilting.
The inventor of the present invention also conditioned a pad using a conditioning plate of the prior art under a flexible joint mechanism. In the conditioning, the inventor of the present invention applied a coating over the polishing pad surface to evaluate the conditioning uniformity based on the amount of the removed coating.
The result of the above evaluation, as shown in FIG. 9, clearly shows that the polishing pad surface was unevenly conditioned using the conventional conditioning mechanism. This obviously means that the polishing pad surface was unevenly polished because the conditioner discontinuously contacted the pad as described above and also the uneven thickness of the polishing pad or uneven attachment of the polishing pad affected the polishing.